1. Field of the Invention
The present invention relates to a transmission circuit and a system for the same, and, more particularly, to a transmission circuit outputting power signal using frequency hopping system and a system having the transmission circuit.
2. Description of Related Art
A device that transmits and receives data using wireless signal outputs power signal from an antenna connected to an output of a transmission circuit through a transmission path. However, in the transmission path, power loss generally occurs. Therefore, even when the signal is output from the transmission circuit in a desired power, power level of the power signal output from the antenna can be attenuated compared with the power level of the power signal output from the transmission circuit.
A related example for compensating the attenuation of the power level occurred at the transmission path is disclosed in Japanese Unexamined Patent Application Publication No. 6-37652. FIG. 5 schematically shows a wireless communication device according to the related example. As shown in FIG. 5, the wireless communication device includes a transmission circuit 101, a transmission path 102, a power sense circuit 103, and an antenna 104. A power amplifier 111 of the transmission circuit 101 amplifies data signals input through a modulator 110 to output the power signal. This power signal passes through the transmission path 102 and the power sense circuit 103 and then the power signal is output from the antenna 104.
The wireless communication device according to the related example detects power level of the power signal output from the antenna 104 by a transmission power detection circuit 122 included in the power sense circuit 103. The transmission power detection circuit 122 receives the power signal output from the antenna 104 through a bifurcation unit 121. Then a direct current voltage obtained at the transmission power detection circuit 122 is transmitted to a comparator 114 through a composite unit 120, the transmission path 102 and bifurcation unit 112. The comparator 114 compares the direct current voltage with reference voltage output from a reference signal generation circuit 113. When it is determined by the comparator 114 that the power signal output from the antenna 104 has smaller power level, the comparator 114 increases the gain of the power amplifier 111. In other words, the wireless communication device according to the related example detects the power loss occurred at the transmission path 102 and increases the gain of the power amplifier 111 according to the detection result, to thereby setting the power level of the power signal output from the antenna 104 to a desired power level.
Japanese Unexamined Patent Application Publication No. 11-340908 discloses a technique for setting power level of the power signal output from the amplifier in advance when the wireless base station device is in non-operating state and for controlling power level of the power signal according to the set value when the wireless base station device is in operating state. However, in the wireless base station device disclosed in Japanese Unexamined Patent Application Publication No. 11-340908, power level output from the amplifier is not set according to the power level of the power signal at the antenna end. Therefore, it is not possible to address power loss of the power signal occurred at the transmission path.
A technique called UWB (Ultra Wide Band) has recently been suggested. UWB is defined by FCC (Federal Communications Commission) as wireless signal having fractional bandwidth of 25% or more or wireless signal having bandwidth of 450 MHz or more. UWB is broadly recognized as a system communicated by the wireless signal. One system example for realizing UWB is MB-OFDM (Multi Band-Orthogonal Frequency Division Multiplexing). FIGS. 6 and 7 schematically show the power signal that is transmitted when MB-OFDM type UWB, in other words MB-OFDM UWB is used. FIG. 6 shows a relationship between frequency bandwidth of power signal transmitted in MB-OFDM UWB and power level for each frequency bandwidth. On the other hand, in MB-OFDM, frequency hopping system is used for hopping frequency of the power signal that is transmitted in a predetermined time interval. By using frequency hopping system, it is possible to output instantaneous electric power three times higher than in the system that uses the same bandwidth but does not perform frequency hopping even when average output power is the same. In other words, by employing MB-OFDM, it is possible to almost double maximum propagation distance when minimum receiver sensitivity is constant. OFDM is used in MB-OFDM. OFDM is a communication system that is suitable for constant wireless transmission or small-scale in-building network because OFDM has high efficiency on frequency axis and has high tolerance against fading. Examples of the communication system employing OFDM include digital terrestrial broadcasting and wireless LAN, for example. In digital terrestrial broadcasting, power signal having bandwidth BW of 6 MHz per channel is used. Wireless LAN based on the IEEE 802.11 standard has bandwidth BW of 18 MHz.
In the system employing frequency hopping, power loss with respect to the power signal that is transmitted is different between frequency bands that are hopped. Therefore, extra function is needed in the device used in this system for compensating power loss that is different between frequency bands. Japanese Unexamined Patent Application Publication No. 9-139694, Japanese Unexamined Patent Application Publication No. 2005-269202, and Japanese Unexamined Patent Application Publication No. 8-316880 disclose techniques for measuring or controlling power level of the power signal for each frequency band.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 9-139694, power level of the power signal is controlled by controlling an attenuator rapidly using control information stored in a memory in advance. In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-269202, the power level of the power signal after being modulated is measured based on a training pattern generated according to a hopping pattern of the power signal. When the data signal is transmitted, the power level of the power signal is controlled according to the measurement result. In the technique disclosed in Japanese Unexamined Patent Application Publication No. 8-316880, the power level of the power signal that is transmitted is detected and displayed.
The power signal used in the MB-OFDM UWB has bandwidth BW of 1584 MHz. In other words, in MB-OFDM UWB, wider bandwidth BW is used compared with the system employing other OFDM systems to transmit the power signal.
As stated above, when the signal having wide bandwidth BW is transmitted through the transmission path, power loss generated at the transmission path greatly varies between low frequency band and high frequency band of the bandwidth BW. Therefore, when the power level of the power signal output from the amplifier is controlled without considering power loss generated at the transmission path as techniques disclosed in Japanese Unexamined Patent Application Publication No. 11-340908, Japanese Unexamined Patent Application Publication No. 9-139694, Japanese Unexamined Patent Application Publication No. 2005-269202, and Japanese Unexamined Patent Application Publication No. 8-316880, the power level of the power signal output from the antenna greatly varies between frequency bands due to power loss occurred at the transmission path.
FIG. 7 schematically shows one example of hopping pattern in which the power signal is hopped in MB-OFDM UWB. As shown in FIG. 7, in MB-OFDM UWB, a switching time of the frequency of the power signal (hereinafter referred to as hopping frequency) is 9.5 nsec. For example, when the gain of the amplifier is controlled while outputting the power signal as disclosed in Japanese Unexamined Patent Application Publication No. 6-37652, time of around several msec is typically needed to control this gain. Therefore, the time for switching the hopping frequencies is so short in MB-OFDM UWB that it is quite difficult to control the power level of the power signal for each hopping frequency by using the technique disclosed in Japanese Unexamined Patent Application Publication No. 6-37652.